In a semiconductor memory device such as an SSD (Solid State Disk), a large number of NAND flash memory packages are electrically connected to a substrate through solder connections. The capacity of each flash memory has increased, and the performance of each flash memory has become more sophisticated in recent years. In this trend, flash memory packages are often in the form of BGAs (Ball Grid Arrays) suitable for high-density packaging. In usage environments, however, many kinds of external forces, such as a thermal load generated by a linear expansion coefficient difference between components, and a load generated by vibration or impact from outside, are applied to the solder joints of BGAs, and there is a constant risk of breakage. Particularly, the solder bumps (hereinafter referred to as bumps) on the outer circumferences of BGAs are most likely to break due to the load generated by heat, vibration, or impact. To prevent easy loss of function of packages due to breakage, those bumps on the outer circumferences are often dummy bumps that are not used for signal lines or power supplies. Such dummy bumps are now expected to be applied to prognostics and health monitoring so that a danger to memory packages can be predicted by detecting breakage in the dummy bumps. JP 10-93297 A discloses a technique for detecting breakage in the dummy bumps at the four corners in a BGA package. Also, JP 2002-76187 A discloses a technique for measuring the resistance of a BGA substrate, and, based on the measurement result, detecting a stressed state among the joints of the BGA substrate.
As described above, a semiconductor memory device such as an SSD includes a large number of semiconductor memory packages. Therefore, the intensity of the mechanical load to be applied varies among the packages, depending on the layout on the substrate. If a certain amount of load is accumulated in a package, functionally-essential solder joints connected to signal lines, power supply lines, and the like are broken, and the information stored in the package is lost. If breakage can be predicted in advance, the information in the package is transferred to another information storage medium when the unsafe level is reached. In this manner, a backup of information can be made before the information is lost.
However, as the number of packages increases, the number of dummy bumps increases. As a result, the time and load required for detection also increase. Also, where RAIDs (Redundant Arrays of Inexpensive Disks) each including SSDs are used, for example, the structure becomes more complicated. As a result, the load applied during the defect detection becomes large, and a long period of time is required.